1. Field of the Invention
The present invention relates generally to a ring laser gyroscope, and specifically to a system and method for providing cavity length control in a ring laser gyroscope in response to sudden shocks and high g-force loads experienced by the ring laser gyroscope.
2. Description of Related Art
A ring laser gyroscope is a wave resonator which sustains multiple laser modes within a ring cavity. A laser light beam is introduced into the e ring cavity at a frequency such than an integral number of wavelengths will fit exactly within the path length of the resonant ring cavity. An integral number of wavelengths around the light beam cavity path of the gyroscope are required to support resonant operation. The ring cavity is filled with an active gaseous medium, typically Helium and Neon, to provide the gain for the laser beam and for sustaining a plurality of laser modes propagating within the ring cavity.
The ring laser gyroscope includes at least one pair of counterpropagating light beams traveling around the resonator path in the ring cavity. A plurality of mirrors form the ring resonator path, where one of the mirrors is slightly transmissive to allow light to leave the resonator. The light passing through the transmissive mirror is appropriately recombined and the intensity of an interference pattern is sensed and processed to compute angular displacement, angular velocity, fringe rate, etc., about a predetermined axis. In order to ensure proper lasing operation, the cavity length must be tuned in such a way that the gaseous medium will provide sufficient gain at the cavity""s resonant frequency.
One manner of controlling cavity length has been accomplished by adjusting the position of at least one of the corner mirrors in the resonant cavity to control the cavity length of the ring laser gyroscope, as taught by U.S. Pat. No. 5,208,653 (issued May 4, 1993) entitled xe2x80x9cMULTIOSCILLATOR RING LASER GYROSCOPE ADAPTIVE DIGITALLY CONTROLLED CAVITY LENGTH CONTROL SYSTEM,xe2x80x9d which was filed by two of the applicants hereto and assigned to the common assignee of this application. In this type of cavity length control system, a piezoelectric transducer having driving electrodes is attached to a movable mirror to change the position of the movable mirror depending upon the voltage delivered to the electrodes. A control servo-loop is used to monitor the operation of the ring laser gyroscope and adjust the position of the movable mirror to maintain the cavity resonance at a desirable gain condition, where a typical servo-loop is illustrated in FIG. 1. The intensity of the laser light passing through the partially transparent mirror is observed by intensity detection sensors 102. A cavity length error detection unit 104 then determines whether the measured intensity deviates from the desired maximum intensity, where a control unit 106 then applies a control algorithm to determine the amount of movement required to adjust the cavity length to achieve the desired intensity condition of the propagating wavelengths. The control unit 106 then instructs a mirror driver amplifier 108 to apply a certain voltage to the piezoelectric transducer 110 in order to provide the appropriate cavity length correction. The piezoelectric transducer 110 then applies a force on the movable mirror in order to appropriately change its position and, in turn, the cavity length.
One significant problem which arises in this type of ring laser gyroscope is that when experiencing sudden shocks or high g force accelerations, the movable mirror and piezoelectric transducer assembly also move relative to the cavity body due to their inertia and the finite stiffness of the assembly. For slowly varying disturbances, the control servo-loop driving the piezoelectric transducer will compensate for the cavity length change by measuring the light passing through a partially transparent mirror and adjusting the voltage applied to the piezoelectric transducer to maintain the propagating wavelengths at the desired intensity condition. Unfortunately, the bandwidth of the control loop is often limited as a result of the cavity length error determination method and the digital signal processing iteration rate utilized. Further, the light intensity measurement itself can be noisy, and this noise would thus be demodulated and applied to the mirror via feedback in the control loop. It is undesirable to have this type of high frequency noise on the mirror, because this would produce noise in the gyroscope output as well.
In order to avoid this problem with noise without increasing the bandwidth through expensive and complicated measures, the control servo-loop typically has a very narrow bandwidth. Due to this narrow bandwidth, it is no possible for the control servo-loop to respond to rapid variations in cavity length which may result from suddenly encountered shocks. As a result, the instantaneous cavity length can change substantially during large shocks or sudden high g force accelerations. If the change in cavity length is large enough, some of the laser modes will have insufficient gain to be resonant which can cause them to drop out. This detrimentally affects the operation of the gyroscope by producing erroneous measurements. Other attempts at resisting high g forces in a ring cavity gyroscope have been directed at making the mirror stiffer to resist movement. However, a stiffer mirror also requires a larger amount of force to be applied by the piezoelectric transducer to move the mirror, wherein conventional piezoelectric transducers are limited in the amount of force which they can generate. Thus, merely stiffening the mirror has not proven to be effective in providing proper cavity length control in high g force environments.
There is clearly a need for a system and method for controlling the cavity length of a ring laser gyroscope in response to sudden shocks or high g forces. Moreover, there is a need for a cavity length control system and method which maintains the cavity resonance of a ring laser gyroscope at a desirable gain condition at all times in a high g-force environment.
The present invention provides a system and method for controlling the cavity length of a ring laser gyroscope to properly tune the resonant wavelengths of the ring laser gyroscope relative to the gain curve during moments when experiencing sudden shocks or high g force accelerations. The system includes a ring laser gyroscope having an optical pathway for light waves propagating among a plurality of mirrors defining the optical pathway, wherein at least one of the mirrors is movable for changing the length of the optical pathway. A mirror driver is provided for moving the movable mirror for purposes of cavity length control. The cavity length control system provides controlled activation of the mirror driver. The system includes at least one accelerometer for measuring the acceleration experienced by the ring laser gyroscope, wherein a compensation signal for activating the mirror driver is generated that is relative to the measured acceleration to counteract the effects of the acceleration on the movable mirror. By adjusting the position of the movable mirror, the wavelengths propagating in the ring laser gyroscope will remain resonant and continue to lase even while experiencing this acceleration.
The cavity length control system may further include a prediction filter for estimating the acceleration which will be measured next by the accelerometer. Since the compensation signal is providing cavity length control for accelerations which have already been measured, the estimated acceleration measurement is used to modify the compensation signal in order to predict the appropriate cavity length correction required to maintain the wavelengths in the ring laser gyroscope resonant in a real-time manner. The system may further include a control servo-loop for monitoring the intensity of the wavelengths propagating within the gyroscope and controlling the position of the movable mirror by producing a servo-loop control signal that is a function of mirror control voltage and the change in light wave intensity as such intensity relates to the mirror driver voltage. The servo-loop control signal is combined with the compensation signal to generate a mirror driver signal which provides activation of the mirror driver for providing both immediate and long-term cavity length control.